(1) In wavelength division multiplexing (WDM) for simultaneously transmitting a plurality of optical signals of different wavelengths, a wavelength division multiplexing apparatus is an important device. A wavelength division multiplexing apparatus has the function of a wavelength multiplexing apparatus for multiplexing optical signals of different wavelengths (a multiplexing function) and the function of a wavelength divider for dividing wavelength-multiplexed light into optical signals of different wavelengths (a wavelength division function).
As a result, a plurality of information data sets are multiplexed by the wavelength division multiplexing apparatus and multiplexed into an optical wavelength band, and wavelength-multiplexed light assigned to channel 1 to channel “n” (“n” denotes a natural number, and a channel represents an optical wavelength). Light of desired channel “i” (“i” denotes a natural number) among the wavelength-multiplexed light is subjected to wavelength division by the wavelength division multiplexing apparatus provided at a relay station or a terminal station. The wavelength-divided optical signal of channel “i” is converted into an electric signal, and desired information data sets are delivered or transferred to a plurality of users.
In the field where the wavelength division multiplexing apparatus is utilized, the wavelength division multiplexing apparatus can be used as, for example, a branch section of an optical fiber for connecting access nodes of a main optical network to respective households, in addition to being used as an optical switch, an ADM (Add & Drop Multiplexing apparatus), or the like. Moreover, the wavelength division multiplexing apparatus is also used for processing an optical signal of a transmission terminal disposed in the household, or for like processing. An attempt can be made to curtail costs of these optical switches by miniaturizing hardware of the switches, and the quality of the optical switches is improved. In order to promote miniaturization of the hardware, the degree of integration of components of the hardware must be increased. In this regard, the wavelength division multiplexing apparatus is important also as an integrated optical component of waveguide type.
(1-1) About Arrayed Waveguide Grating (Hereinafter Referred to as an “AWG” Unless Otherwise Specified).
A plurality of types of wavelength division multiplexing apparatus have been known, and attention is given to AWG as a practical device (element) among these pieces of wavelength division multiplexing apparatus. This AWG uses an arrayed waveguide grating formed from an optical waveguide provided on a chip-like planer substrate.
FIG. 13 is a block diagram of the AWG. The AWG shown in FIG. 13 is formed from a substrate (e.g., a silicon substrate) 1 and an optical waveguide collectively fabricated on the substrate 1 through use of photolithography. The AWG is formed from an input waveguide 2; an input slab (an input-side slab waveguide) 300; an arrayed waveguide (a channel waveguide array) 400 made from seven (seven channels) channel waveguide (channel light waveguide) groups “a” to “g”; an output slab (an output-side slab waveguide) 500; and an output waveguide 6 made from seven waveguides “h” to “n.” The seven channel waveguides “a” to “g” have the function of causing a phase difference and are called a phased array. The number of channel waveguides “a” to “g” is seven. However, this number can be set to a desired value.
As a result, the wavelength-multiplexed light input to the input waveguide 2 from the outside by way of, e.g., an optical fiber, is spread (diffused) by means of diffraction of the input slab 300. The thus-spread lights are input to (enter) the respective seven channel waveguides “a” to “g” constituting the arrayed waveguide 400 by way of seven slab connection sections (connection sections, connection portions) 9.
FIG. 14 is an enlarged view of the input slab 300 and the channel waveguide array 400. The slab connection sections 9 correspond to bases (roots) of the respective channel waveguides “a” to “g” shown in FIG. 14 and are seven locations on a slab boundary. An interval d1 between the channel waveguides corresponds to a distance between two adjacent slab connection sections 9. Moreover, the channel waveguide interval d1 usually becomes shorter with decreasing distance from a center 310 (the spreading center of incident light) achieved when incident light undergoes diffraction to an incident position of the channel waveguide array 400 (this distance will be hereinafter referred to as a “slab length”).
The lengths of the seven channel waveguides “a” to “g” shown in FIG. 13 (waveguide lengths) differ from each other. Wavelength-multiplexed lights #1 to #7 entering the arrayed waveguide 400 are in phase with each other. However, the wavelength-multiplexed lights #1 to #7 are out of phase with each other at the output side of the arrayed waveguide 400 in accordance with a difference between the lengths of the seven channel waveguides “a” to “g.”
The wavelength-multiplexed lights #1 to #7 are output (radiated) from the respective channel waveguides “a” to “g” and interfere with each other. The thus-output wavelength-multiplexed lights are gathered in (converged on) the vicinity of the slab connection section 9 existing between the output slab 500 and the output waveguide 6. Positions where single lights of respective wavelengths included in the wavelength-multiplexed lights #1 to #7 (hereinafter called “single lights”) are gathered change from one wavelength to another. For this reason, components of wavelength 1 included in the wavelength-multiplexed lights #1 to #7 from the channel waveguide “a” are gathered in a channel waveguide “h” among seven channel waveguides “h” to “n” constituting the output waveguide 6, by way of the output slab 5. Components of the respective wavelengths 2 to 7 are gathered in the channel waveguides “i” to “n.” Accordingly, the wavelength-multiplexed lights #1 to #7 are divided into single lights #1 to #7, so that the wavelength division function is exhibited. The gathered seven single lights are input into the seven channel waveguides “h” to “n” of the output waveguide 6 and guided to the end of the substrate 1.
The seven single lights are input into the output waveguide 6 and transferred in the direction opposite that mentioned previously. As a result, the wavelength-multiplexed lights #1 to #7 are obtained from the input waveguide 300, thereby exhibiting the multiplexing function.
As mentioned above, the AWG realizes an operation analogous to that of a spectrometer using a long-known diffraction grating, through use of the channel waveguides “a” to “g” collectively fabricated on the substrate 1. Therefore, the AWG is considered to be a promising, compact wavelength division multiplexing apparatus for wavelength multiplexing communication which is conducive to mass production.
Generally, the multiplexing function can be realized by rendering the input and output directions of light opposite to the input and output directions of light achieved at the time of wavelength division. Unless otherwise specified, the following descriptions are provided while attention is paid chiefly to the wavelength division function among the two functions.
(1-2) About a Method for Plotting the Geometry of the Channel Waveguide Array 400 of Seven Channel Waveguides
The geometry of the channel waveguide array 400 is plotted such that a difference between the lengths of adjacent channel waveguides, such as the channel waveguides “c” and “d,” among the seven channel waveguides “a” to “g” becomes constant.
FIG. 15 is a view for describing basic methods for plotting a conventional channel waveguide array, wherein the view consists of an upper row, a middle row, and a lower row. An arrayed grating optical division multiplexing apparatus provided in the upper row of FIG. 15 is formed such that an interval between waveguides of a channel waveguide array at a boundary between a first fan-shaped slab waveguide and the channel waveguide array becomes different from an interval between waveguides of a channel waveguide array at a boundary between the channel waveguide array and a second fan-shaped slab waveguide (e.g., Patent Document 1).
According to the plotting method provided in the upper row, the lengths of the respective channel waveguides are adjusted by adjusting the lengths Li of linear portions (linear sections) of the vertical channel waveguide. As a result, the degree of freedom of design can be increased, and a device capable of effecting highly-functional optical frequency separation and optical frequency wavelength multiplexing can be obtained.
In an optical wavelength division multiplexing apparatus provided in the middle row of FIG. 15, the lengths of respective arrayed waveguides of an arrayed waveguide diffraction grating are adjusted such that an error between phases of light waves developing in the arrayed waveguide diffraction grating is substantially eliminated (e.g., Patent Document 2).
According to the plotting method provided in the middle row of FIG. 15, the lengths of the respective channel waveguides are adjusted by adjusting the length Li of a linear portion of the channel waveguide connected to the slab, the curvature radius Ri of a circular arc, and a central angle θi of the circular arc. Thereby, an error between the amounts of changes in phases of the light waves traveling through the arrayed waveguide becomes small, and crosstalk deterioration due to a side lobe, or the like, is dampened.
An arrayed waveguide grating provided in the lower row of FIG. 15 is for connecting two output waveguides, one being longer than the other, to a first fan-shaped slab waveguide with an interval which is half that existing between output channel waveguides, wherein the two output waveguides belong to input channel waveguides including two waveguides having a predetermined optical path length difference and two 3 dB couplers constituting an asymmetrical Mach-Zehnder interferometer coupling both ends of the two waveguides (e.g., Patent Document 3).
According to the plotting method provided in the lower row of FIG. 15, the lengths of the respective channel waveguides are adjusted by adjusting the length Li of a linear portion of the channel waveguide connected to the slab, the curvature radius Ri of a circular arc, a central angle θi of the circular arc, and a length L2i of a linear portion of a channel waveguide connected to the circular arc.
As a result, a flat band which is half an interval between channels or more is obtained, and there is realized a flat optical frequency characteristic whose wavelength division output characteristic remains essentially constant even when the wavelengths of the light source have changed.
As mentioned above, a variety of geometries or structures for the AWG have hitherto been proposed, and the above-described three types of geometries have been known as the basic methods for plotting a channel waveguide.
A wavelength division multiplexing apparatus capable of handling add-and-drop function is also known (e.g., Patent Document 4). This wavelength division multiplexing apparatus is constituted such that a waveguide grating router and a phase-shifter cause reflection. By virtue of this configuration, the result of a test on transmission of light over 16 channels at 100 GHz using InP (indium phosphorus) is improved.
Moreover, an AWG which realizes high wavelength isolation at low cost is also known (e.g., Patent Document 5).
(1-3) About Miniaturization of the AWG
In accordance with expansion of utilization of wavelength division multiplexing, a compact AWG suitable for mass production is desirable. Therefore, demand has hitherto existed for lowering the costs of chips by increasing the number of chips per wafer (also called a “yield” and corresponds to a proportion of non-defective chips to manufactured chips) by means of reducing a chip size (the size of the substrate 1 having waveguides fabricated thereon, the size of a substrate on which waveguides have not yet been fabricated, or the area of the substrate). Another desire also exists for diminishing a loss in a waveguide and lessening a loss in the slab connection section 9 by means of miniaturizing the chip size.
FIG. 16 is a view for describing a relationship between the channel waveguide interval d1 and the slab length f1 in the slab connection section 9. A ratio of the channel waveguide interval d1 to the slab length f1 in the slab T1 shown in FIG. 16; that is, d1/f1, and a ratio of a channel waveguide interval d11 to a slab length f′ in a slab T2 shown in FIG. 16; that is, d11/f′, are constant (‘/’ denotes a division operation). Accordingly, in order to reduce the slab length f1 of the slab T1, the channel waveguide interval d1 in the slab connection section 9 must be reduced.
(1-4) However, signal lights (separated light) propagating through the respective channel waveguides “a” to “g” of the AWG all shift to any of the adjacent channel waveguides “a” to “g” while propagating through the channel waveguides “a” to “g,” whereby distortion of phase information (phase difference) and deteriorating characteristics of respective signal lights are induced, to thus cause so-called crosstalk.
Therefore, broadening the channel waveguide interval d1 so as to reduce optical coupling between the channel waveguides “a” to “g” is indispensable. Particularly, a phase difference between any of the adjacent channel waveguides in an area (a region) other than the neighborhood of the slab connection section 9 is larger than that achieved in the vicinity of the slab connection section 9. Hence, the influence of optical coupling existing between the channel waveguides “a” to “g” is noticeable. Accordingly, there is adopted a waveguide structure where the channel waveguide intervals “a” to “g” become larger with increasing distance from the slab connection section 9.
In general, the channel waveguide interval d1 in the slab connection section 9 used in the AWG and the channel waveguide interval d11 in the area most distant from the slab connection section 9 change according to a difference Δn between a specific refraction factor of the core of the channel waveguide and a specific refraction factor of the clad of the same. For instance, in the case of a channel waveguide of Δn=0.75%, factors of d1=14 μm to 20 μm and d11=30 to 40 μm are used.
The core is an area where the AWG is fabricated and which is made from material whose refractive index is higher than that of an area surrounding the AWG. The clad is an area which surrounds the core and is made from material whose refractive index is lower than that of the core.
As mentioned above, when the adjacent channel waveguides are arranged over a long distance while being separated from each other by only a small interval d1, the adjacent channel waveguides cause cross-talk or a transmission loss, thereby deteriorating characteristics of an optical signal.
Accordingly, in consideration of deterioration of characteristics stemming from coupling between the channel waveguides, the channel waveguide interval d1 or d11 in the slab connection section 9 of the conventional AWG cannot be reduced. Therefore, there exists a problem of an increase in the slab length f1 or f′ and an increase in the chip size of the AWG. Put another way, when the channel waveguide interval d1 or d11 in the slab connection section 9 is reduced for diminishing the chip size of the AWG, deterioration, such as cross-talk or a transmission loss, arises.
The AWGs described in Patent Documents 1 to 5 are all not intended for reducing a chip size.
Patent Document 1
Japanese Patent Laid-Open No. HEI 11-2733
Patent Document 2
Japanese Patent Laid-Open No. 2000-352630
Patent Document 3
Japanese Patent Laid-Open No. HEI10-90530
Patent Document 4
U.S. Pat. No. 6,141,467
Patent Document 5
Japanese Patent Laid-Open No. 2001-174653